A Half-Delivered Promise

Employees at a Nissan plant in Aguascalientes, Mexico, put the finishing touches on car bodies

The first day of the new year will mark the 20th anniversary of the North American Free Trade Agreement, or NAFTA, a deal that was meant to transform struggling Mexico by greatly increasing trade with the U.S. and Canada. Did it work? Mexico is certainly a far better country today  but NAFTA is not the only source of this change.

Seen exclusively as a trade deal, it has been a success. Mexico's exports leaped from $60 billion in 1994 to nearly $400 billion today. These exports are mainly manufactured goods, like...